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Circadian Science
Warm Light and Sleep: Why Warm White Isn't Night-Ready
The LONVIA
Edit
Curated insights on space, health, investment, and the future of living.
Circadian Science
Warm Light and Sleep: Why Warm White Isn't Night-Ready
10 min read
Warm white describes how a light looks, not what it does to your body clock. Warming and dimming an ordinary bulb lowers the biological signal, but it does not switch it off, because most warm sources still carry the short-wavelength content that the clock reads. A room can look like evening while your body has not been told the day is over.
Warm does not mean night-ready
What “warm white” actually is, and why your body reads it differently to your eye.
Step outside on a clear morning and your eyes meet something in the order of ten thousand lux, climbing past a hundred thousand under direct summer sun. Step back indoors and that figure collapses. A well-lit office sits around three to five hundred lux. A sitting room in the evening, lamps on and curtains drawn, often runs below two hundred. To the eye the difference is modest, because the eye adjusts quickly and asks for little. To the system in the body that keeps time, the difference is the entire point.
We have inherited a simple rule for managing that indoor world. Bright and cool for the day, warm and dim for the evening. Lower the lights, warm the colour, and the body is meant to follow. It is the oldest advice there is about light and sleep, and it is not wrong so much as incomplete. Warm describes how a light looks. It says very little about what a light does.
What warm white actually is
When a manufacturer labels a bulb warm white, the claim is about colour. Warm light leans amber and gold, cool light leans blue and white, and the scale between them has a name: colour temperature, described in the same way a heated metal glows first red, then orange, then white as it grows hotter. Lower on the scale looks cosy. Higher looks clinical.
That scale is a description of appearance, and appearance is drawn from human vision. Human vision turns out to be a poor guide to biology, because the eye and the body clock do not weight light the same way. Daytime vision is most sensitive to green light, in the middle of the spectrum, which is why lux, the standard measure of brightness, is built around that green-weighted response. The clock is tuned elsewhere. It answers most strongly to the blue-white light of an open sky. A warm bulb can therefore look calm to your eye and still carry a signal your clock reads as daylight, because the two are measuring different things.
The reader most people have never heard of
For most of the last century the retina was thought to hold two kinds of light detector: rods for dim light, cones for colour and detail. Both serve vision. Then, at the turn of the millennium, a third was confirmed. Ignacio Provencio and colleagues identified a light-sensitive pigment, melanopsin, in the inner retina (Provencio et al., 2000). The groups of David Berson and Samer Hattar then showed that a small population of retinal ganglion cells carrying it are photosensitive in their own right, firing in response to light with no help from rods or cones (Berson et al., 2002; Hattar et al., 2002).
These cells, the intrinsically photosensitive retinal ganglion cells, do little for the images we see. Their fibres run instead to the suprachiasmatic nucleus, the master clock in the hypothalamus, and onward to the circuitry that governs the pineal gland's release of melatonin, the hormone that marks biological night. They are, in effect, the body's light meter for timekeeping rather than seeing. Their pigment is most sensitive to the short-wavelength, blue part of daylight, the very region a warm bulb is designed to reduce but seldom removes.
When these cells register bright, blue-rich light, the clock reads day and melatonin stays down. When that light falls away, the clock reads evening and melatonin rises. So the question that matters at night is not whether the room looks golden. It is whether this pathway has been told the day is over.
Why warm and dim only gets you halfway
Here the old rule begins to fail, and the evidence against it is not subtle.
The human clock is far more sensitive to light than its daytime tolerance suggests. In a controlled study of night-time exposure, Jeanne Zeitzer and colleagues found that around one hundred lux, roughly the level of a dimly lit room, produced half of the maximum suppression of melatonin, with the response saturating by about two hundred lux (Zeitzer et al., 2000). The ceiling for melatonin disruption sits at light levels most people would call comfortable.
Later work sharpened the point. Joshua Gooley's team showed that ordinary room light of less than two hundred lux, in the hours before bed, shortened the body's nightly melatonin window by around ninety minutes compared with dim light, and that light during sleep itself suppressed melatonin by more than half (Gooley et al., 2011). Reading on a light-emitting screen before bed, in work by Anne-Marie Chang and colleagues, suppressed evening melatonin by roughly fifty-five per cent and pushed the circadian clock later, leaving readers less alert the following morning (Chang et al., 2015).
Two features of this evidence matter for the warm-and-dim rule. The first is that sensitivity varies enormously between people. Andrew Phillips and colleagues found that the light level producing half-maximal melatonin suppression differed by more than fifty-fold across healthy adults, meaning a setting that is safely dim for one person is well past the threshold for another (Phillips et al., 2019). The second is that warming and dimming address only part of the problem. A warm bulb carries less of the short-wavelength, blue light the clock responds to than a cool one, and dimming lowers the total. But less is not none, brightness multiplies whatever remains, and typical evening lighting still clears the low bar these studies describe. The room looks ready for sleep. The pathway is not convinced.
Measuring the thing that matters
If colour and brightness are the wrong instruments for the evening, a better one already exists.
Lux weights light by the sensitivity of daytime vision, that green-centred response. It answers the question how bright does this look. It was never built to answer how strongly does this speak to the clock. To close the gap, researchers defined a parallel measure built around the melanopsin response rather than the visual one (Lucas et al., 2014), formalised in 2018 as an international standard, CIE S 026. The metric is melanopic equivalent daylight illuminance, or mEDI, and it states a light in terms of how much daylight-equivalent signal it delivers to the clock.
The distinction is not academic. Two lamps can read identically in lux and diverge sharply in mEDI, because one holds more of the short-wavelength content the clock detects. One can look, to the eye, like a calm evening, and register, to the body, like late morning. This is the foundation of circadian lighting: designing to the melanopic measure rather than the visual one.
It also lets guidance be written in the body's own units. In 2022 a group of leading circadian researchers drew the available evidence together and proposed target ranges for healthy adults: at least two hundred and fifty mEDI through the day, no more than ten mEDI in the three hours before bed, and under one mEDI during sleep (Brown et al., 2022). Note what is absent from those figures. Not a word about how warm or cool the light should look. The recommendation is written in signal, not in appearance.
Warm was never wrong. It was just never the whole story.
None of this makes warm light a mistake. A warm, low-lit room in the evening is pleasant, and set against a bright cool one it is a real improvement. The instinct behind the old rule points in the right direction.
But the rule was built to change how a room looks, and the body was never consulted. When Kenneth Wright's group sent people camping with no light but the sun and the fire, their clocks advanced and tightened to the solar day within a week, then drifted late again once ordinary indoor lighting returned (Wright et al., 2013). We did not evolve under lamps. The lighting that fills our evenings is a recent arrangement, optimised for mood and for vision, and only lately examined for what it does to physiology.
That examination is where the category has to go next. Visual comfort and biological support are different measurements, and a softer-looking room is not evidence of a better one. The colour on the outside of the light was never the signal. The signal sits underneath, in a part of the spectrum the eye barely notices and the clock cannot ignore.
The next standard for evening light will not be judged by how the room looks to the person sitting in it. It will be judged by the one reader that decides whether they sleep well. Not the eye. The clock.
Common questions
Does warm light help you sleep?
It helps more than bright, cool light, but it does not finish the job. Warming and dimming a bulb lowers the signal reaching the body clock; it does not remove it. Controlled studies show that light levels most people find comfortable in the evening are still enough to suppress melatonin and delay the clock (Zeitzer et al., 2000; Gooley et al., 2011).
What colour of light is best before bed?
Colour is the wrong variable. Two lights of the same warm appearance can deliver very different signals to the clock, depending on their spectrum and their brightness. The measure that tracks the biological effect is mEDI, not colour temperature (CIE S 026; Brown et al., 2022).
Is blue light the only problem at night?
Blue-rich light has the strongest effect on the clock, but it is not the whole story. Total brightness matters, timing matters, and sensitivity differs widely from person to person (Phillips et al., 2019). A dim warm light and a bright warm light are not equivalent, and neither can be judged by colour alone.
References
Provencio, I., Rodriguez, I. R., Jiang, G., Hayes, W. P., Moreira, E. F., Rollag, M. D. (2000). A novel human opsin in the inner retina. Journal of Neuroscience, 20(2), 600-605.
Berson, D. M., Dunn, F. A., Takao, M. (2002). Phototransduction by retinal ganglion cells that set the circadian clock. Science, 295(5557), 1070-1073.
Hattar, S., Liao, H. W., Takao, M., Berson, D. M., Yau, K. W. (2002). Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science, 295(5557), 1065-1070.
Zeitzer, J. M., Dijk, D. J., Kronauer, R. E., Brown, E. N., Czeisler, C. A. (2000). Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression. The Journal of Physiology, 526(3), 695-702.
Gooley, J. J., Chamberlain, K., Smith, K. A., et al. (2011). Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. The Journal of Clinical Endocrinology & Metabolism, 96(3), E463-E472.
Chang, A. M., Aeschbach, D., Duffy, J. F., Czeisler, C. A. (2015). Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proceedings of the National Academy of Sciences, 112(4), 1232-1237.
Wright, K. P., McHill, A. W., Birks, B. R., et al. (2013). Entrainment of the human circadian clock to the natural light-dark cycle. Current Biology, 23(16), 1554-1558.
Phillips, A. J. K., Vidafar, P., Burns, A. C., et al. (2019). High sensitivity and interindividual variability in the response of the human circadian system to evening light. Proceedings of the National Academy of Sciences, 116(24), 12019-12024.
Lucas, R. J., Peirson, S. N., Berson, D. M., et al. (2014). Measuring and using light in the melanopsin age. Trends in Neurosciences, 37(1), 1-9.
Brown, T. M., Brainard, G. C., Cajochen, C., et al. (2022). Recommendations for daytime, evening, and night-time indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biology, 20(3), e3001571.
CIE S 026/E:2018. CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light. International Commission on Illumination, Vienna.
Roksana Fasovska
LONVIA Founder
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Ready to shape a healthier future?
Book an introductory conversation to explore how well-being can be integrated into your space or project
Ready to shape a healthier future?
Book an introductory conversation to explore how well-being can be integrated into your space or project
10 min read
June 10, 2024
Warm does not mean night-ready
What “warm white” actually is, and why your body reads it differently to your eye.
Step outside on a clear morning and your eyes meet something in the order of ten thousand lux, climbing past a hundred thousand under direct summer sun. Step back indoors and that figure collapses. A well-lit office sits around three to five hundred lux. A sitting room in the evening, lamps on and curtains drawn, often runs below two hundred. To the eye the difference is modest, because the eye adjusts quickly and asks for little. To the system in the body that keeps time, the difference is the entire point.
We have inherited a simple rule for managing that indoor world. Bright and cool for the day, warm and dim for the evening. Lower the lights, warm the colour, and the body is meant to follow. It is the oldest advice there is about light and sleep, and it is not wrong so much as incomplete. Warm describes how a light looks. It says very little about what a light does.
What warm white actually is
When a manufacturer labels a bulb warm white, the claim is about colour. Warm light leans amber and gold, cool light leans blue and white, and the scale between them has a name: colour temperature, described in the same way a heated metal glows first red, then orange, then white as it grows hotter. Lower on the scale looks cosy. Higher looks clinical.
That scale is a description of appearance, and appearance is drawn from human vision. Human vision turns out to be a poor guide to biology, because the eye and the body clock do not weight light the same way. Daytime vision is most sensitive to green light, in the middle of the spectrum, which is why lux, the standard measure of brightness, is built around that green-weighted response. The clock is tuned elsewhere. It answers most strongly to the blue-white light of an open sky. A warm bulb can therefore look calm to your eye and still carry a signal your clock reads as daylight, because the two are measuring different things.
The reader most people have never heard of
For most of the last century the retina was thought to hold two kinds of light detector: rods for dim light, cones for colour and detail. Both serve vision. Then, at the turn of the millennium, a third was confirmed. Ignacio Provencio and colleagues identified a light-sensitive pigment, melanopsin, in the inner retina (Provencio et al., 2000). The groups of David Berson and Samer Hattar then showed that a small population of retinal ganglion cells carrying it are photosensitive in their own right, firing in response to light with no help from rods or cones (Berson et al., 2002; Hattar et al., 2002).
These cells, the intrinsically photosensitive retinal ganglion cells, do little for the images we see. Their fibres run instead to the suprachiasmatic nucleus, the master clock in the hypothalamus, and onward to the circuitry that governs the pineal gland's release of melatonin, the hormone that marks biological night. They are, in effect, the body's light meter for timekeeping rather than seeing. Their pigment is most sensitive to the short-wavelength, blue part of daylight, the very region a warm bulb is designed to reduce but seldom removes.
When these cells register bright, blue-rich light, the clock reads day and melatonin stays down. When that light falls away, the clock reads evening and melatonin rises. So the question that matters at night is not whether the room looks golden. It is whether this pathway has been told the day is over.
Why warm and dim only gets you halfway
Here the old rule begins to fail, and the evidence against it is not subtle.
The human clock is far more sensitive to light than its daytime tolerance suggests. In a controlled study of night-time exposure, Jeanne Zeitzer and colleagues found that around one hundred lux, roughly the level of a dimly lit room, produced half of the maximum suppression of melatonin, with the response saturating by about two hundred lux (Zeitzer et al., 2000). The ceiling for melatonin disruption sits at light levels most people would call comfortable.
Later work sharpened the point. Joshua Gooley's team showed that ordinary room light of less than two hundred lux, in the hours before bed, shortened the body's nightly melatonin window by around ninety minutes compared with dim light, and that light during sleep itself suppressed melatonin by more than half (Gooley et al., 2011). Reading on a light-emitting screen before bed, in work by Anne-Marie Chang and colleagues, suppressed evening melatonin by roughly fifty-five per cent and pushed the circadian clock later, leaving readers less alert the following morning (Chang et al., 2015).
Two features of this evidence matter for the warm-and-dim rule. The first is that sensitivity varies enormously between people. Andrew Phillips and colleagues found that the light level producing half-maximal melatonin suppression differed by more than fifty-fold across healthy adults, meaning a setting that is safely dim for one person is well past the threshold for another (Phillips et al., 2019). The second is that warming and dimming address only part of the problem. A warm bulb carries less of the short-wavelength, blue light the clock responds to than a cool one, and dimming lowers the total. But less is not none, brightness multiplies whatever remains, and typical evening lighting still clears the low bar these studies describe. The room looks ready for sleep. The pathway is not convinced.
Measuring the thing that matters
If colour and brightness are the wrong instruments for the evening, a better one already exists.
Lux weights light by the sensitivity of daytime vision, that green-centred response. It answers the question how bright does this look. It was never built to answer how strongly does this speak to the clock. To close the gap, researchers defined a parallel measure built around the melanopsin response rather than the visual one (Lucas et al., 2014), formalised in 2018 as an international standard, CIE S 026. The metric is melanopic equivalent daylight illuminance, or mEDI, and it states a light in terms of how much daylight-equivalent signal it delivers to the clock.
The distinction is not academic. Two lamps can read identically in lux and diverge sharply in mEDI, because one holds more of the short-wavelength content the clock detects. One can look, to the eye, like a calm evening, and register, to the body, like late morning. This is the foundation of circadian lighting: designing to the melanopic measure rather than the visual one.
It also lets guidance be written in the body's own units. In 2022 a group of leading circadian researchers drew the available evidence together and proposed target ranges for healthy adults: at least two hundred and fifty mEDI through the day, no more than ten mEDI in the three hours before bed, and under one mEDI during sleep (Brown et al., 2022). Note what is absent from those figures. Not a word about how warm or cool the light should look. The recommendation is written in signal, not in appearance.
Warm was never wrong. It was just never the whole story.
None of this makes warm light a mistake. A warm, low-lit room in the evening is pleasant, and set against a bright cool one it is a real improvement. The instinct behind the old rule points in the right direction.
But the rule was built to change how a room looks, and the body was never consulted. When Kenneth Wright's group sent people camping with no light but the sun and the fire, their clocks advanced and tightened to the solar day within a week, then drifted late again once ordinary indoor lighting returned (Wright et al., 2013). We did not evolve under lamps. The lighting that fills our evenings is a recent arrangement, optimised for mood and for vision, and only lately examined for what it does to physiology.
That examination is where the category has to go next. Visual comfort and biological support are different measurements, and a softer-looking room is not evidence of a better one. The colour on the outside of the light was never the signal. The signal sits underneath, in a part of the spectrum the eye barely notices and the clock cannot ignore.
The next standard for evening light will not be judged by how the room looks to the person sitting in it. It will be judged by the one reader that decides whether they sleep well. Not the eye. The clock.
Circadian Science
Warm Light and Sleep: Why Warm White Isn't Night-Ready
10 min read
June 10, 2024
Warm does not mean night-ready
What “warm white” actually is, and why your body reads it differently to your eye.
Step outside on a clear morning and your eyes meet something in the order of ten thousand lux, climbing past a hundred thousand under direct summer sun. Step back indoors and that figure collapses. A well-lit office sits around three to five hundred lux. A sitting room in the evening, lamps on and curtains drawn, often runs below two hundred. To the eye the difference is modest, because the eye adjusts quickly and asks for little. To the system in the body that keeps time, the difference is the entire point.
We have inherited a simple rule for managing that indoor world. Bright and cool for the day, warm and dim for the evening. Lower the lights, warm the colour, and the body is meant to follow. It is the oldest advice there is about light and sleep, and it is not wrong so much as incomplete. Warm describes how a light looks. It says very little about what a light does.
What warm white actually is
When a manufacturer labels a bulb warm white, the claim is about colour. Warm light leans amber and gold, cool light leans blue and white, and the scale between them has a name: colour temperature, described in the same way a heated metal glows first red, then orange, then white as it grows hotter. Lower on the scale looks cosy. Higher looks clinical.
That scale is a description of appearance, and appearance is drawn from human vision. Human vision turns out to be a poor guide to biology, because the eye and the body clock do not weight light the same way. Daytime vision is most sensitive to green light, in the middle of the spectrum, which is why lux, the standard measure of brightness, is built around that green-weighted response. The clock is tuned elsewhere. It answers most strongly to the blue-white light of an open sky. A warm bulb can therefore look calm to your eye and still carry a signal your clock reads as daylight, because the two are measuring different things.
The reader most people have never heard of
For most of the last century the retina was thought to hold two kinds of light detector: rods for dim light, cones for colour and detail. Both serve vision. Then, at the turn of the millennium, a third was confirmed. Ignacio Provencio and colleagues identified a light-sensitive pigment, melanopsin, in the inner retina (Provencio et al., 2000). The groups of David Berson and Samer Hattar then showed that a small population of retinal ganglion cells carrying it are photosensitive in their own right, firing in response to light with no help from rods or cones (Berson et al., 2002; Hattar et al., 2002).
These cells, the intrinsically photosensitive retinal ganglion cells, do little for the images we see. Their fibres run instead to the suprachiasmatic nucleus, the master clock in the hypothalamus, and onward to the circuitry that governs the pineal gland's release of melatonin, the hormone that marks biological night. They are, in effect, the body's light meter for timekeeping rather than seeing. Their pigment is most sensitive to the short-wavelength, blue part of daylight, the very region a warm bulb is designed to reduce but seldom removes.
When these cells register bright, blue-rich light, the clock reads day and melatonin stays down. When that light falls away, the clock reads evening and melatonin rises. So the question that matters at night is not whether the room looks golden. It is whether this pathway has been told the day is over.
Why warm and dim only gets you halfway
Here the old rule begins to fail, and the evidence against it is not subtle.
The human clock is far more sensitive to light than its daytime tolerance suggests. In a controlled study of night-time exposure, Jeanne Zeitzer and colleagues found that around one hundred lux, roughly the level of a dimly lit room, produced half of the maximum suppression of melatonin, with the response saturating by about two hundred lux (Zeitzer et al., 2000). The ceiling for melatonin disruption sits at light levels most people would call comfortable.
Later work sharpened the point. Joshua Gooley's team showed that ordinary room light of less than two hundred lux, in the hours before bed, shortened the body's nightly melatonin window by around ninety minutes compared with dim light, and that light during sleep itself suppressed melatonin by more than half (Gooley et al., 2011). Reading on a light-emitting screen before bed, in work by Anne-Marie Chang and colleagues, suppressed evening melatonin by roughly fifty-five per cent and pushed the circadian clock later, leaving readers less alert the following morning (Chang et al., 2015).
Two features of this evidence matter for the warm-and-dim rule. The first is that sensitivity varies enormously between people. Andrew Phillips and colleagues found that the light level producing half-maximal melatonin suppression differed by more than fifty-fold across healthy adults, meaning a setting that is safely dim for one person is well past the threshold for another (Phillips et al., 2019). The second is that warming and dimming address only part of the problem. A warm bulb carries less of the short-wavelength, blue light the clock responds to than a cool one, and dimming lowers the total. But less is not none, brightness multiplies whatever remains, and typical evening lighting still clears the low bar these studies describe. The room looks ready for sleep. The pathway is not convinced.
Measuring the thing that matters
If colour and brightness are the wrong instruments for the evening, a better one already exists.
Lux weights light by the sensitivity of daytime vision, that green-centred response. It answers the question how bright does this look. It was never built to answer how strongly does this speak to the clock. To close the gap, researchers defined a parallel measure built around the melanopsin response rather than the visual one (Lucas et al., 2014), formalised in 2018 as an international standard, CIE S 026. The metric is melanopic equivalent daylight illuminance, or mEDI, and it states a light in terms of how much daylight-equivalent signal it delivers to the clock.
The distinction is not academic. Two lamps can read identically in lux and diverge sharply in mEDI, because one holds more of the short-wavelength content the clock detects. One can look, to the eye, like a calm evening, and register, to the body, like late morning. This is the foundation of circadian lighting: designing to the melanopic measure rather than the visual one.
It also lets guidance be written in the body's own units. In 2022 a group of leading circadian researchers drew the available evidence together and proposed target ranges for healthy adults: at least two hundred and fifty mEDI through the day, no more than ten mEDI in the three hours before bed, and under one mEDI during sleep (Brown et al., 2022). Note what is absent from those figures. Not a word about how warm or cool the light should look. The recommendation is written in signal, not in appearance.
Warm was never wrong. It was just never the whole story.
None of this makes warm light a mistake. A warm, low-lit room in the evening is pleasant, and set against a bright cool one it is a real improvement. The instinct behind the old rule points in the right direction.
But the rule was built to change how a room looks, and the body was never consulted. When Kenneth Wright's group sent people camping with no light but the sun and the fire, their clocks advanced and tightened to the solar day within a week, then drifted late again once ordinary indoor lighting returned (Wright et al., 2013). We did not evolve under lamps. The lighting that fills our evenings is a recent arrangement, optimised for mood and for vision, and only lately examined for what it does to physiology.
That examination is where the category has to go next. Visual comfort and biological support are different measurements, and a softer-looking room is not evidence of a better one. The colour on the outside of the light was never the signal. The signal sits underneath, in a part of the spectrum the eye barely notices and the clock cannot ignore.
The next standard for evening light will not be judged by how the room looks to the person sitting in it. It will be judged by the one reader that decides whether they sleep well. Not the eye. The clock.